The present invention relates generally to an automatic selector device for a multi-gear change-speed gearbox in which are provided gear selectors actuatable by selector actuators of axial piston type in order to form a desired gear, an electronic control unit, a multi-path valve common to the selector actuators and an electromotive valve actuator operative in response to the control unit for actuating the multi-path valve, the multi-path valve having for each gear, an associated valve position in which the respective selector actuator is connected to an operating pressure source.
In a known selector device of the type shown in DE 27 44 286 A1, operating pressure is switched on and off directly by way of a multi-path selector valve common to all the selector actuators. The multi-path selector valve is driven into its valve positions associated with a particular gear by a stepper motor whose operation depends on the electronic control unit. Since large pressure medium flows are necessary for the selector actuators in the case of rapid selecting movements, the known multi-path selector valve is, among other things, of large and heavy design because of the flow cross-sections necessary so that the use of an electromotive stepper motor also becomes problematical.
U.S. Pat. No. 3,274,858 shows an automatic selector device of a different type, i.e. an epicyclic gearbox with eight forward gears and four reverse gears for driving an agricultural tractor. Two plate clutches are located in the force path between an input shaft which can be driven by a drive engine, and the epicyclic gearbox and a third plate clutch is located between the epicyclic gearbox and an output shaft leading to the engine. Three plate brakes are each connected to an outer central wheel of the epicyclic gearbox, and a fourth plate brake is connected to an inner central wheel of the epicyclic gearbox. Of this total of seven selector devices, each of which is actuated by way of a selector actuator having a single-acting axial piston, the selector actuators of the two input-end plate clutches are each connected to a separate selector valve. A 2/6-way valve is used for the selector valve of the one input-end plate clutch which is engaged in the two lowest and the two highest forward gears. A 2/5-way valve is used for the selector valve of the other input-end plate clutch, which is engaged in the six upper forward gears, and is directly connected to a reservoir pressure conduit emerging from a pressure reservoir. The 2/6-way selector valve is connected via a switch-off valve which can be actuated by a clutch pedal to a supply pressure conduit which charges the pressure reservoir and which is supplied by a pressure pump. A 2/5-way selector valve common to the two selector actuators of two plate brakes each connected to an outer central wheel is supplied by a pressure conduit which emerges from the 2/6-way selector valve of the single input-end plate clutch and which is led via the 2/5-way selector valve of the other input-end plate clutch and is then in open connection with the part of the supply conduit emerging from the switch-off valve when both input-end plate clutches are disengaged. A 2/5-way selector valve is again common to the two selector actuators of the output-end third plate clutch and the third plate brake connected to the inner central wheel, which 2/5-way selector valve, in turn, is not directly connected to the section of the reservoir pressure conduit connecting the switch-off valve to the pressure reservoir but via a 3/4-way selector valve of the axial spool type. As a departure from this arrangement, no special selector valve is provided for the selector actuator of the fourth plate brake but, when one of the reverse gears is engaged, the operating pressure is supplied to this selector actuator via the 3/4-way selector valve. In this known selector device, therefore, a 2/6-way selector valve and three 2/5-way selector valves are used for three plate clutches and three plate brakes. These valves can be reversed as a function of a second selector valve, i.e. a 9/6-way valve of the axial spool type. The multi-path valve is directly supplied by the supply pressure conduit and is manually actuated jointly with the first-mentioned 3/4-way selector valve. With this known selector device, an attempt was made to permit power shifts without interruption to the tractive force in agricultural operation with short selection times and rapid driving manoeuvres between forward and reverse motion.
An automatic hydraulic selector device for change-speed gearboxes is shown in DE 19 08 914 A1, and uses a pressure medium pump, a selector pressure valve connected to the pressure medium pump, several gear selection valves connected in parallel with the selector pressure valve, pair of friction clutches which can be engaged and disengaged alternately by the gear selection valves and a valve device subjected to pressure in the friction clutch to be engaged. By way of the valve device, the ventilation of the friction clutch to be disengaged is controlled as a function of the rise in pressure in the friction clutch to be engaged. A selector pressure valve can be influenced by the engine torque to maintain a selector pressure in the pressure conduit which varies with the engine torque and a hydraulic double-acting valve device for closing and opening purposes can be controlled, on one hand, by the pressure drop in the friction clutch to be disengaged, and by the pressure rise in the friction clutch to be engaged, and, on the other hand, by the variable selection pressure. In this manner, the pressure, in the friction clutch which has to be engaged, at which the valve device is opened and the friction clutch which has to be disengaged is ventilated, is matched to the pressure (transmission pressure) at which the friction clutch to be engaged transmits the particular load torque. The gear selection valves can be actuated individually by an electromagnetic actuator, and an attempt is made to control the overlapping disengagement and engagement of the friction clutches during a gear change such that an interruption to the drive torque is avoided. For this purpose, the friction clutches should be actuated by variable transmission pressures which depend on the engine torque to be transmitted.
An object on which the present invention is based comprises the creation of an automatic selector device of a multi-gear change-speed gearbox with gear-wheels which can be disconnected from their shaft, in which large pressure medium flows and a high pressure level can be made available for rapid selection movements of the selector actuators without a multiplicity of electromagnetic control valves being necessary for this purpose because the flow cross-sections of the latter are, in general, relatively small.
The foregoing object has been achieved by the present invention in an advantageous manner by the provision of a 3-position cylinder with a double-acting axial piston constitutes the selector actuator for two adjacent gears, and a 4/3-way selector valve is arranged to switch on and off operating pressure of each 3-position cylinder, the 4/3-way selector valve being actuatable by a control pressure force supplied via the multi path valve constituting a control valve, and each selector valve is independently connected to a main pressure conduit which can be at a high pressure level whereas the multi-path valve is connected to an auxiliary pressure conduit independent with respect to pressure level of the main pressure conduit.
In the selector device according to the present invention, it is now possible to use selector spool valves which can be configured with large flow cross-sections and are controlled by a common multi-path control valve operating with small control flows, and therefore of small size, without the necessity of solenoid valves.
Moreover, a conventional stepper motor of relatively low torque can be used advantageously as the valve actuator for actuating the multi-path control valve, and a conventional 3-position cylinder with a double-acting axial piston can be provided to actuate in each case two of the relevant synchronizing clutches.
Another feature according to the present invention is a 4/3-way selector valve which is used for switching on and off the operating pressure in the two operating pressure chambers of the respective 3-position cylinder.
According to the present invention, it is, in fact, possible to use a multi-path control valve of the axial spool type. In order to avoid the use of a transmission for converting a rotary motion into a linear motion, it is advantageous in the selector device according to the invention for the drive of the multi-path control valve by way of a stepper motor for this control valve to be configured as a rotary spool valve.
It is also advantageous in accordance with the present invention, when using an intrinsically low-torque stepper motor, for the motor to operate on the multi-path control valve via an intermediate transmission.
In a particularly advantageous embodiment of the multi-path control valve of rotary spool type of the present invention, an angular position and a valve housing plane normal to an axis of rotation of the rotary spool valve are associated with the rotary spool of the multi-path control valve for each gear, and in each valve housing plane, a valve inlet connected to a pressure source, a substantially unpressurized valve outlet, a valve outlet connected to a control pressure conduit associated with the gear and a control duct of the rotary spool are associated such that the valve outlet connected to the control pressure conduit is connected via the control duct in the associated angular position to the valve inlet whereas, in other angular positions, the valve outlet is connected to the unpressurized valve outlet.